Electrodialytic production of stannic oxide sol

ABSTRACT

STANNIC OXIDE SOL IS PRODUCED BY ELECTRODIALYTICALLY TRANSFERRING METAL CATIONS OF A WATER-SOLUBLE STANNATE, SUCH AS POTASSIUM STANNATE, FROM THE ANODE COMPARTMENT OF AN ELECTROLYTIC CELL TO THE CATHODE COMPARTMENT WHILE SIMYLTANEOUSLY SUBSTANTIALLY PREVENTING MIGRATION OF TIN ANIONS FROM THE ANODE COMPARTMENT TO THE CATHODE COMPARTMENT BY MAINTAINING A CATION PERMSELECTIVE DIALYTIC MEMBRANE BETWEEN THE ANODE AND THE CATHODE.

United States Patent 3,723,273 ELECTRODIALYTIC PRODUCTION OF STANNIC OXIDE SOL Harold P. Wilson, Sewickley, Pa., assignor to Vulcan Materials Company, Birmingham, Ala. No Drawing. Filed Sept. 24, 1971, Ser. No. 183,713 Int. Cl. B01d 13/02 US. Cl. 204-180 P 8 Claims ABSTRACT OF THE DISCLOSURE Stannic oxide sol is produced by electrodialytically transferring metal cations of a water-soluble stannate, such as potassium stannate, from the anode compartment of an electrolytic cell to the cathode compartment While simultaneously substantially preventing migration of tin anions from the anode compartment to the cathode compartment by maintaining a cation permselective dialytic membrane between the anode and the cathode.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to the electrodialytic production of stannic oxide sol.

Summary of the prior art Stannic or tin oxide sols are known and are useful in tin electrodeposition processes. As described in US. Pat. 3,346,468, tin oxide sol has heretofore been produced by an involved process which includes reacting an alkali metal stannate with an acid to precipitate hydrous stannic oxide, washing the hydrous stannic oxide to remove undesirable ions, and peptizing the hydrous stannic oxide. As described in US. Pat. 3,455,794, tin oxide sol has also been produced by contacting an aqueous solution of alkali metal stannate with particular cation exchange resins at limited rates of solution-resin contact and with necessary regeneration of exhausted cation exchange resm.

The search has continued, however, for more direct stannic sol processes which would reduce the number of major steps involved; which would eliminate the need for neutralization or regeneration by acid reagents, and which would eliminate the need for Wash solutions and their virtually inherent inefiiciencies.

SUMMARY OF THE INVENTION Accordingly, a primary object of the present invention is to provide for the production of stannic oxide sols without incurring or substantially alleviating the problem heretofore associated with the production of such sols.

Another object of the present invention is to provide an electrodialytic process for the production of stannic oxide sol without significantly incurring tin plating of the cathode and contamination of the catholyte by tin anions, and which process is also characterized by relatively low power requirements and the ability to recover and reuse electrolyte.

Another object of the present invention is to provide stannic oxide sols produced by such a process.

These and other objects will become apparent to one skilled in the art from the following:

In accordance with a primary aspect of the present invention, stannic oxide sol is produced by electrodialyticaly transferring or transporting metal cations of a Water-soluble. stannate from an anode compartment of an electrolytic or electrodialytic cell to a cathode compartment of the cell and simultaneously substantially preventing migration of tin anions from the anode compartment to the cathode compartment by maintaining a ice 5 will become apparent to one skilled in the art from the following description of the preferred embodiments:

DESCRIPTION OF THE PREFERRED EMBODIMENTS The electrolytic cell used in accordance with the present invention to produce the stannic oxide sol typically includes at least one anode compartment containing at least one anode immersed in a liquid electrolyte, hereinafter referred to as the anolyte, and at least one cathode compartment containing at least one cathode immersed in a liquid electrolyte, hereinafter referred to as the catholyte. The anode and the cathode are separated by a cation permselective membrane.

The anode and cathode are connected to a suitable power source at their terminals; and whenever electric potential is applied at the terminals, stannic oxide sol is produced in the anode compartment. Oxygen gas is also produced in the anode while hydrogen gas is produced at the cathode. These gases may be vented out of the system by any conventional means as is well known to those skilled in the art.

The anode and cathode compartments may be provided with agitators or stirrers, and heating or cooling means may also be provided to maintain the anolyte and the catholyte at the desired operating temperatures.

The electrolytic cell may be operated at anode current densities of about 5 to 100, and more desirably 20 to 50 amperes per square foot of anode area, and at cell voltages ranging from about 1 to 20 volts, and preferably from about 1 to 10 volts.

The term anode area as used herein is defined as the cross-sectional area of the immersed frontal area of the anode, facing the cathode.

The temperature of the anolyte may be from about 5 C. up to about C., but more typically is about 10 C. to 30 C., and preferably is from about 15 C. to 25 C., to avoid phase separation in the anolyte. The temperature of the catholyte may fall within the same ranges given for the anolyte, and preferably is within about 5 C. of the anolyte temperature.

Of course, one may employ a multiple cell operation, i.e., anode compartments interposed between cathode compartments. For example, the electrolytic cell may comprise two stainless steel cathodes with a single interposed stainless steel anode, with a cation permselective membrane separating each of the cathodes from the anode.

The anode and cathode may be of any convenient shape such as a sheet or rod, preferably sheet, and the overall size of the anode, the cathode, and their respective compartments may be varied according to the particular scale of operation.

Any type of anode and cathode material that is electrically conductive and has low reactivity in the electro lyte solution, i.e., is substantially inert to the electrolyte, may be used. For example, sheets or panels of iron or steel may be used. Stainless steel has been found to be particularly suitable.

The anolyte, i.e., the electrolyte in the anode compartment, may be any aqueous solution of a water-soluble metal stannate. Typically, the anolyte is an aqueous solution of an alkali metal stannate. Mixtures of more than one stannate may also be used, e.g., a mixture of potassium and sodium stannates may be used, but more typically only one stannaate is used at any one time.

Aqueous solutions of potassium stannate are preferred as the anolyte.

The concentrations of stannate in the anolyte may range from about g.p.l. (expressed or measured herein as total grams of tin present per liter of solution) up to saturation, i.e., an aqueous solution saturated with watersoluble stannate at operation temperatures. Preferably, aqueous potassium stannate solutions in concentrations of about 100 g.p.l. up to saturation, and most preferably from above about 250 g.p.l. up to saturation, are used.

The catholyte, i.e., the electrolyte in the cathode compartment, may be any aqueous solution of a water-soluble metal hydroxide. Typically, the metal or cation of the hydroxide in the catholyte is the same metal or cation of the stannate in the anolyte, and preferably the water-soluble metal hydroxide in the catholyte is an alkali metal hydroxide. Mixtures of more than one hydroxide may also be used, e.g., a mixture of potassium and sodium hydroxides may be used, but more typically only one hydroxide is used at any one time.

Aqueous solutions of potassium hydroxide or sodium hydroxide are preferred as the catholyte.

The concentration of hydroxide in the catholyte may range up to a concentration slightly less than saturation, i.e., an aqueous solution not completely saturated with water-soluble metal hydroxide at operating temperatures. Preferably, the catholyte concentration is weaker or less concentrated in potassium ions than the anolyte to facilitate the electrodialysis process.

Aqueous potassium hydroxide or sodium hydroxide solutions may, for example, be used in concentrations of from about 10 g.p.l. (expressed or measured herein as total grams of metal hydroxide present per liter of solution) up to about 250 g.p.l.

In general, any type of cation permselective membrane may be used which will substantially exclude or prevent tin anions from passing from the anode compartment to the cathode compartment of the electrolytic cell, but which will allow passage of cations therethrough.

Typically, the cation permselective membrane is a cation exchange membrane or sheet which is substantially impermeable to the aqueous electrolyte. These cation exchange membranes are well known per se and include both membranes where ion exchange groups or material are impregnated in or distributed throughout a polymeric matrix or binder, as well as those where such groups are associated only with the outer surface of a membrane backing or reinforcing fabric. Continuous ion exchange membranes, in which the entire membrane structure has ion-exchange characteristics and which may be formed by molding or casting a partially polymerized ion exchange resin into sheet form, may also be used.

For example, the ion exchange material may include material to which acid groups such as -SO H or -COOH are added to a polystyrene resin by conventional procedures. In the alternative, the groups may be added by contacting the surface to be coated with a reactant, the molecular structure of which leaves exposed to the surface thereof ion exchange groups of the same type as those found upon the surfaces of cation exchange membranes, e.g., -SO H or -COOH radicals.

Widely known cation exchange membranes may be prepared by copolymerizing a mixture of ingredients, one of which contains a substituent or group which is acid in nature and which may comprise the sulfonic acid group or the carboxylic acid group. Thus, this ionizable group may be attached to a polymeric compound such as copolymers of styrene and divinyl benzene, polystyrene phenolaldehyde resins, resorcinol-aldehyde polymers, copolymers of divinyl benzene with acrylic acid, copolymers of divinyl benzene with maleic anhydride, copolymers of divinyl benzene with acrylonitrile, copolymers of divinyl benzene and methacrylic acid, cellulose derivatives such as regenerated cellulose, ethyl cellulose and polyvinyl alcohol, and like polymers containing free hydroxyl groups, which are reacted with sulfonating agents, and polyethylene reacted with chlorosulfonic acids or other sulfonating agents.

The preparation of these cation exchange membranes are well known in the art and for sake of brevity are not further described herein; for more detailed information, reference may be made to US. Pats. 2,681,320, 2,723,229, 2,832,728, 3,113,911, 3,356,607 and 3,480,495, all of which are incorporated herein by reference.

In addition to these organic cation exchange membranes inorganic ion exchange membranes may also be used. A description of inorganic ion exchange membranes may be found in US. Patent 3,463,713, which is also incorporated herein by reference.

Typically, these ion exchange membranes are reinforced, i.e., have a backing consisting of a sheet of a relatively inert material, as for example, glass having a woven or mesh structure. Other known backings include woven and non-woven fabrics of materials such as asbestos, polyesters, polyamides, acrylics, modacrylics, ceramic or glass fibers, vinylidene chloride, rayons, polypropylene, polytetrafiuoroethylene and the like. Fabrics or backings made of mixtures of two or more of these materials may also be used in the present invention.

The thickness of the cation permselective membrane is not particularly critical, but will of course depend on the particular operating conditions. In general, suitable membranes may be as thin as 20,000th of an inch to as much as /2 inch thick. The minimum thickness of a membrane will also depend on the total thickness of the supporting structure. Although the thicker membranes have a longer useful life, their electrical resistances increase proportionally to their thickness, so that if the membrane is made increasingly thicker, a value will be obtained for which the resistance is too great for practical use.

Typical commercially available cation exchange membranes include those available from Ionics Incorporated, Watertown, Mass; from Ionac Chemical Company, Birmingham, N.J., under the trade name Ionac, and from AMF Incorporated of New York, N.Y., under the trade name AMFion.

The present invention may be conducted on a batch, semi-continuous, or continuous basis and at atmospheric, superatmospheric or subatmospheric pressures, typically at atmospheric pressure.

The present invention is further illustrated by the following examples; all parts, percentages and ratios in the examples, as well as in other parts of the specification and claims, are by weight unless otherwise specified.

EXAMPLE This example illustrates the production of stannic oxide sol using an aqueous potassium stannate anolyte, an aqueous potassium hydroxide catholyte, and a cation permselective membrane in accordance with the present 1nvention.

A run Was made using an electrolytic cell rectangular in cross section and whose walls and base were fabricated from 1.75 cm. thick Plexiglas poly(methyl methacrylate) acrylic sheet. Dividing this cell into anode and cathode compartments was a fabric-backed cation exchange membrane composed of a sulfonated copolymer of styrene and divinyl benzene, number MC-3470 from the Ionac Chemical Company, of Birmingham, N]. This cation exchange membrane was strongly cation permselective having a 96.2% cation permselectivity measured in a 0.5 N NaCl/ 1.0 N NaCl cell. This membrane was also substantially impermeable to electrolyte flow, as it passed less than 10 ml. H O/hl'./ft. at 30 p.s.i. and less than 5 ml. H O/hr./ft. at 10 p.s.i. This membrane was approximately 13 to 14 mils thick, and had an approximate density of 405 g./m. with a Mullen burst strength of p.s.i. This membrane also had an electrical resistance of 9.6 ohm-cmF, A.C. measurement in 0.1 N NaCl.

The total volumetric capacity (working solution capacity) of the electrolytic cell was 2.5 liters approximately evenly divided by the membrane between the anode and cathode compartments. The width of the cell at the membrane was 12.8 cm. Stainless steel sheets were used as the anode and cathode, and hydrogen gas produced at the cathode was vented away from the system. The anode was spaced 6.5 cm. away from the membrane and the cathode was spaced 3.5 cm. away from the membrane.

The run conducted was a batch operation except that the catholyte was diluted with additional water throughout the run by replacing a portion of the potassium hydroxide solution with deionized water, and the anolyte at approximately the mid-point of the run, i.e., halfway through the run, received additional potassium stannate equivalent in tin to about 15% of the original anolyte feed so as to maximize the tin content of the final stannic oxide sol product. Also, the anolyte was cooled by means of a conventional heat exchanger so as to maintain the desired operating temperature. The anode compartment of the electrolytic cell was also supplied with a motor driven glass propeller for agitation.

It was noted during the run that the anolyte shrank in volume since, apparently, water as well as potassium ions were transferred through the membrane to the cathode compartment.

Data and results for the run are shown in the following table.

PRODUCTION OF STANNIC OXIDE SOL Electrochemical process values Total electrodialysis time, min 3,043 Average temperature of the anode solution, C. 19.5 Current consumption, amp-hr. 155.28 Average current, amperes 3.06 Average anode current density amp/ft. frontal area 38.18 Average membrane current density, amp/ft. 29.07 Average cell potential, volts 7.43 Average electrodialytic current efficiency, percent 99.42 Power consumption, k.w. h./lb. sol produced 0.350 Production rate, lb. sol./hr.-ft. membrane 0.618 Total tin in system, g. 442.7 Percent of tin in system plated on cathode 0.11 Percent of tin in system transferred through membrane 1.87

treated sol with the aid of a mild swirling action retained fluidity with no phase separation or any other noticeable change. When maintained at 3 C., the stannic oxide sol product remained clear and quite fluid throughout. As an elevated temperature test, the stannic oxide sol product was agitated mildly in a small erlenmeyer flask over a period of 5.75 hours as the temperature was gradually increased from 24 C. to 89.5 C. and then allowed to gradually drop to 61.8 C. During this time period there was no change in fluidity or appearance of the stannic oxide sol product. It was also calculated that the run achieved a 103.82% conversion to stannic oxide sol product. The extent of conversion of potassium stannate to stannic oxide sol was calculated on the basis that theoretically 75% of the tin in the potassium stannate was converted into the metastannic acid sol by virtue of removal of potassium by the electrodialysis to form a stable coordinated complex structure such as H K[Sn(OH) The formation of such a complex structure was reflected by acceleration of specific solution resistance as well as overall potential and anode overvoltage at near 100% conversion. The stannic oxide sol product was found to be completely soluble in hot potassium stannate electrolytic solutions and could replace electroplated tin with no deleterious effect on the quality of the tin plate produced, as in tin plating processes similar to those described in US. Pat. 3,455,794.

The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variation and changes may be made by those skilled in the art without departing from the spirit of the present invention.

I claim:

1. An electrodialytic process for the production of stannic oxide sol, which process comprises electrodialytically transferring metal cations of a water-soluble stannate from an anode compartment of an electrolytic cell to a COMPOSITION OF ANODE AND CATHODE SOgITgNS INCLUDING FINAL STANNIO OXIDE SOL P D T Solution Final anode (stannic oxide I Initial anode Initial cathode sol product) Final cathode Solution volume, ml.

G.p.l. Percent G.p.l. Percent G.p.l Percent G.p.l. Percent Element or com ound:

Total Sm? 307. 19 19. 59 0. 0 0.0 473. 29. 34 0.57 0.06 48. 1 6. 23 41. 15 4. 00 24. 0 0.09 40. 61 3.05

1 3,007 ml. of catholyte removed from compartment during run and replaced with an equivalent volume of deionized water As "may be seen from the above, very little tin was plated out on the cathode and the concentration of tin in the final cathoode solution was very low. Further, in the run the average electrodialytic current efliciency was better than 99%, and the final stannic oxide sol product was substantially free from uncombined or free potassium hydroxide. According to the following heat stability test made with the stannic oxide sol product recovered from the anode compartment after the run, the stannic oxide sol product was stable and fluid over a wide range of temperatures (2.5 C. to 89.5 C.). In the heat stability test, the stannic oxide sol product was frozen solid at l1 C. and then rewarmed to about 25 C. The thus cathode compartment of the electrolytic cell and simultaneously substantially preventing migration of tin anions from the anode compartment to the cathode compartment by maintaining a cation permselective dialytic membrane between the anode and the cathode, and recovering stannic oxide sol product from the anode compartment of the electrolytic cell.

2. The process of claim 1 wherein an aqueous solution of alkali metal stannate is present in the anode compartment of the electrolytic cell and an aqueous solution of alkali metal hydroxide is present in the cathode compartment of the electrolytic cell.

3. The process of claim 2 wherein the alkali metal stanmate is potassium stannate and the alkali metal hydroxide is potassium hydroxide.

4. An electrodialytic process for the production of stannic oxide sol, which process comprises:

providing an aqueous anoolyte selected from the group consisting of potassium stannate and sodium stannate solutions in contact with an anode in an anode compartment of an electrolytic cell; providing an aqueous anolyte selected from the group consisting of potassium hydroxide and sodium hydroxide solutions in contact with a cathode in a cathode compartment of the electrolytic cell;

applying direct current to the anode and cathode to electrodialytically transfer potassium or sodium cations from the anode compartment to the cathode compartment, and simultaneously substantially preventing migration of tin anions between the anode and the cathode by maintaining an electrolyte fluid-impermeable cation exchange membrane as a cation permselective dialytic membrane between the anode and the cathode to form a stannic oxide sol product in the anode compartment; and

recovering the stannic oxide sole products from the anode compartment. 5. The process of claim 4 further comprising maintaining the anolyte at a temperature of about 10 C. to 30 C., and maintaining a current density of about 5 to about 100 amperes per square foot of anode area.

6. An electrodialytic process for the production of stannic oxide sol, which process comprises:

providing an aqueous potassium stannate anolyte at a concentration of from about 100 g.p.l. to saturation in contact with a substantially insoluble anode in an anode compartment of an electrolytic cell;

maintaining the potassium stannate anolyte at a temperature of from 15 C. to 25 C.;

providing an aqueous potassium hydroxide catholyte at a concentration less than saturation in contact with a substantially insoluble cathode in a cathode compartment of the electrolytic cell;

applying direct current at a current density of about 20 to amperes per square foot of anode area to the anode and cathode to electrodialytically transfer potassium cations from the anode compartment to the cathode compartment and simultaneously substantially preventing migration of tin anions from the anode compartment to the cathode compartment by maintaining an electrolyte fluid-impermeable cation exchange resin membrane as a cation permselective dialytic membrane between the anode and the cathode to form a stannic oxide sol product substantially free of free potassium hydroxide in the anode compartment of the electrolytic cell with substantially no accumulation of tin anions in the cathode compartment and substantially no plating of tin metal on the cathode; and

removing the stannic oxide sol product from the anode compartment of the electrolytic cell.

7. The process of claim 6 wherein the cathoylte is maintained at a temperature of about 15 C. to 25 C. and at a potassium hydroxide concentration of from about 10 up to about 250 g.p.l.; and wherein the anolyte is maintained at a potassium stannate concentration of from about 250 g.p.l. to saturation.

8. The stannic oxide sol product produced by the process of claim 1.

References Cited UNITED STATES PATENTS 2,606,148 8/1952 Portanova et al 20496 2,667,454 1/1954 Roller 20496 3,113,911 12/1963 Jones 204 P X 3,438,879 4/1969 Kircher et a1. 204180 P X 3,455,794 7/1969 Passal et a1. 204-54 R JOHN H. MACK, Primary Examiner A. C. PRESCOTT, Assistant Examiner US. Cl. X.R. 20496 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 273 Dated March 27, 197

I Harold P. Wilson It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected. as shown below:

In the Claims Column 7, line 5, change anoolyte" to anolyte line 9, change "anolyte" to -catholyteline delete "sole products" and insert sol product I Column 8, line 17, change "cathoylte" to catholyte Signed and sealed this, 27th day of November 1973.

(SEAL) Attest:

EDWARD M .FLETCHER,JR. RENE TEGTMEYER Atte'sting Officer I Acting Commissioner of Patents oam 90-1050 (10-69) 

